After following the serial-killer plot of the original Scandinavian series last season, the FX drama (returning today, 9 p.m.) expands to explore the sociopolitical subjects of drug cartels, immigration and poverty along the U.S.-Mexican border and the personal relationships of American detective Sonya Cross (Diane Kruger) and her Mexican counterpart, Marco Ruiz (Demian Bichir).

Sonya and Marco, who partnered to investigate last season's cross-border killings, are estranged as Season 2 starts. Sonya is dealing with the aftershocks of her sister's murder, and Marco "is right in the bottom of hell," Bichir says. "When your son dies and then you lose your wife and daughters, then you are truly fearless. Marco has nothing to lose anymore."

Eventually, the two investigators reunite after a cartel member is found dead in the U.S. But they haven't made peace. "My relationship with Marco is really being tested because I come to understand his involvement with the cartels," Kruger says.

Eleanor Nacht (Franka Potente), a shunned Mennonite who works for cartel boss Fausto Galvan (Ramon Franco), is a key addition. "She's basically an accountant for the cartel, and her payment is revenge," Potente says. "She doesn't care about money or dying, which makes her exceptionally dangerous."

Over the course of this season, Eleanor eventually comes into contact with the investigators, while reporters Daniel Frye (Matthew Lillard) and Adriana Mendez (Emily Rios) follow a money trail that will intersect with law enforcement.

"The ambition this year is to tell a big, multi-part story that has thriller and mystery elements and to see if we can hook the audience," says executive producer Elwood Reid. "It's about an ecosystem."

Some elements, however, remain the same: "There is a lot of blood," Kruger says. "There's a lot of dead people."